The present invention relates generally to a Doppler-based traffic radar system and, more particularly to a radar system and related method for determining the speed of target vehicles independent of the direction of the target vehicles relative to a patrol vehicle supporting the radar system.
Law enforcement officers have utilized Doppler-based traffic radar systems to monitor vehicle speeds and enforce traffic speed limit laws for many years. Throughout this period of time, numerous improvements in both the underlying technology and in the specific application of new processing techniques for the traffic radar systems themselves, have afforded law enforcement officers greater flexibility and improved reliability in carrying out their duties. One such improvement evident in most traffic radar systems presently being marketed includes the capability to more accurately and reliably monitor the speed of certain vehicles while the patrol vehicle is either in a stationary or a moving mode of operation. In fact, the most recent traffic radar systems can now successfully monitor the speed of vehicles approaching the moving patrol vehicle in an opposite lane.
In addition to these capabilities, such radar systems also provide the law enforcement officer the capability to monitor a group of target vehicles simultaneously and to determine the fastest vehicle within the group and/or the vehicle presenting the traditional strongest reflected return signal. Importantly, each of these new or improved capabilities provide the law enforcement officer with a more complete picture of the traffic environment and thus, a more flexible and reliable basis for making more informed decisions.
Despite all the improvements in both the underlying technology utilized in these traffic radar systems and the radar systems themselves, however, a broadly recognized shortcoming of these systems involves the successful monitoring of the speed of vehicles approaching or receding away from the patrol vehicle in the same-lane moving mode without operator intervention. In practice, even the most recent traffic radar systems require the law enforcement officer to make an initial discretionary decision regarding the relative movement of a target vehicle operating in the same lane as the patrol vehicle.
In other words, the law enforcement officer must visually observe the target vehicle in front of or behind the patrol vehicle and decide whether the vehicle is approaching or receding relative to the patrol vehicle. Of course, these manual operations are conducted while also maintaining operation of the patrol vehicle. Additionally, the officer must continually update this information by manually inputting it into the radar system. Typically, a two-position manual switch located adjacent a display or a handheld remote control device of the radar system is utilized to dictate the approaching/receding status of the target vehicle and to indicate the requisite method of calculation to be utilized by the radar system in determining the speed of the target vehicle. These discretionary decisions and required manual operations contribute to the curtailment of the overall flexibility and reliability afforded law enforcement officers by both conventional and digital signal processing (DSP) capable radar systems.
Thus, while conventional and DSP traffic radar systems are both capable of a high degree of accuracy with regard to vehicle speed measurements, great care must be exercised in the use of such systems in properly and accurately attributing a speed to a particular target vehicle. This is of increased importance when monitoring the speed of vehicles approaching or receding away from the patrol vehicle in the same-lane when the law enforcement officer must initially visually observe and determine the direction of the target vehicle relative to the patrol vehicle.
Yet another broadly recognized shortcoming of these traffic radar systems includes the inability to accurately monitor the speeds of vehicles approaching or receding away from the patrol vehicle in the stationary mode of operation. Typically to overcome this shortcoming, the patrol vehicle must position itself between the driving lanes; for example, within the median area along interstates or other divided highways. This effectively eliminates the unwanted approaching or receding targets from the beam of the radar system, thus improving the accuracy and reliability afforded the law enforcement officer. The requirement that the patrol vehicle be located in a particular manner, however, severely limits the overall effectiveness of the radar system.
One alternative to establishing a monitoring position within the median of divided roadways presently available to law enforcement officers is to position the patrol vehicle on the shoulder of the roadway parallel to the highway. This alternative is satisfactory in some settings, particularly along isolated, low volume highways but is of only limited value along the busier and more frequently patrolled highways, such as main traffic arteries. When monitoring vehicle speeds along these highways, the law enforcement officer is forced to wait for openings or gaps in the traffic pattern before utilizing the radar system to determine the speed of approaching/receding opposing lane vehicles. This is due to the potential interference in the radar system processing created by intervening vehicles traveling in the same lane in which the patrol vehicle is positioned.
Accordingly, a need is clearly identified for a radar system and related methods capable of determining the actual speed of selected target vehicles traveling in the same lane as the moving patrol vehicle, or in a different lane than the stationary patrol vehicle. Such a radar system and related methods would effectively remove the reliance on discretionary decisions made by law enforcement officers in determining the relative direction of target vehicles operating in the same lane as the patrol vehicle, and would further increase the overall flexibility and reliability afforded the system operator, particularly in monitoring traffic across busy divided highways.
A Doppler-based traffic radar system is provided that increases the overall flexibility and reliability afforded the system operator, and most importantly eliminates the reliance on discretionary decision making. More specifically, the radar system of the present invention is adapted to determine the speed of at least one moving target traveling in generally the same direction as a platform supporting the radar system independent of the direction of the at least one target relative to the platform. This allows the speed of target vehicles traveling in the same lane and in the same direction as the patrol vehicle to be monitored without manual intervention by the operator. The radar system is further adapted in a stationary mode of operation to determine the speed of a selected moving target independent of the location of the patrol vehicle. This is accomplished in part by selectively eliminating all unwanted approaching or receding targets depending upon the present traffic/patrol scenario.
The radar system of the present invention splits the return signals reflected from the at least one moving target in order to form a test processing signal and a reference processing signal. More specifically, the return signals received from an antenna are split in a turnstile, thus generating first and second split signals. The split signals are substantially equivalent both containing information about the at least one target. A mixer within the turnstile mixes the first split signal with a first portion of the transmitted signal to form a test processing signal. Likewise, the second split signal is mixed with a second portion of the transmitted signal to form a reference processing signal.
Any suitable means may be used to insure that the first and second portions of the transmitted signal, and thus the resultant processing signals, are different in phase. For example, use of microstrip transmission lines of differing lengths connecting each of a pair of sniffer probes for coupling portions of the transmitted signal and mixers is sufficient to cause the signals to be different in phase. Alternatively, a phase shifter or other means known in the art may be utilized.
Because the resulting processing signals are equal in magnitude, but different in phase, the direction of the at least one target relative to the platform can be determined and the speed accurately calculated. For instance, if the phase of the reference signal for an approaching target leads the phase of the test signal, then the phase of the reference signal for a receding target will lag the phase of the test signal. This phase differential in the processing signals also allows the radar system to more accurately calculate the speed of a selected target in the stationary mode of operation.
In accordance with an important aspect of the present invention, speed determining circuitry includes a first means for transforming the processing signals from the time domain to the frequency domain to provide a frequency spectrum, a second means for selectively filtering the processing signals to allow only a range of frequencies substantially centered about the frequency of the selected moving target to pass, and a third means for selectively shifting the phase of one of the processing signals.
As is well known in the art, the resulting frequency spectrum of a time to frequency domain transformation necessarily includes a plurality of bins indicative of the Doppler components of the at least one target. More specifically, the Doppler components include the amplitude and frequency of each target. During operation, the radar system searches the Doppler components and selects a target according to the present operating mode of the radar system. For example, in a strongest target mode of operation, the Doppler components may be searched and a target having the highest amplitude selected. Similarly, in a fastest target mode of operation, the Doppler components may be searched and a target having the highest frequency selected.
Once the step of selecting a target in the moving mode of operation is accomplished, and irrespective of the fastest/strongest mode of operation of the radar system, a second means of the speed determining circuitry may selectively filter the processing signals to allow only a range of frequencies substantially centered about the frequency of the selected moving target to pass. In accordance with one method of the present invention, each of the processing signals are passed through a selected band pass filter to filter unwanted noise and any additional unselected targets including the patrol vehicle signal. The resulting filtered processing signals are further utilized to determine the direction of the selected moving target relative to the platform. Based on the determined direction of the selected moving target, the speed of the target is calculated and displayed by the radar system for the operator""s use.
In addition, a third means of the speed determining circuitry is provided for selectively shifting the phase of one of the processing signals dependent upon a mode of operation of the radar system. Specifically, shifting the phase of one of the processing signals eliminates either all approaching or receding vehicles from further speed calculations when the radar system is in the stationary mode of operation. This allows the law enforcement officer to position the patrol vehicle at any convenient location, for example, along the shoulder of a roadway even if the roadway is a divided highway.
In accordance with another embodiment of the present invention and related method, the direction of the at least one target relative to the platform is determined by cross-correlating the components for each of the bins in the frequency spectrums. Preferably, the processing signals are each transformed into the frequency domain to provide the frequency spectrums as described above. The resulting Doppler components of the at least one frequency spectrums are subsequently cross-correlated by the speed determining circuitry to obtain a cross correlation spectrum of real and imaginary cross-correlated components. In accordance with an important aspect of the present invention, the resulting components of the cross-correlation are indicative of the direction and speed of the at least one target.
During operation, the radar system selects a target from the cross-correlation components according to the present operating mode of the radar system. In a moving, strongest target mode of operation, for example, the cross-correlation components may be searched for a target having the highest amplitude. Similarly, in a moving, fastest target mode of operation, the cross-correlation components may be searched for a target having the highest frequency. Additional modes of operation of radar systems are generally well known in the art and may require the radar system to determine the speed of a target in a stationary, strongest mode; a stationary, fastest mode; a stationary, strongest and fastest mode; a moving, strongest, opposite lane mode; a moving, strongest, same lane mode; a moving, fastest, opposite lane mode; a moving, fastest and strongest, opposite lane mode; a moving, fastest, same lane mode; a moving, fastest, same lane mode; or a moving fastest and strongest same lane mode.
Importantly, the step of selecting at least one target may be made after the direction of the at least one target is determined. Once the step of selecting a target is accomplished, the speed of the at least one target may be calculated dependent upon a determined direction of the target and displayed by the radar system for the operator""s use. Advantageously, this allows for the speed of multiple targets to be determined and displayed.
Still other objects of the present invention will become readily apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of one of the modes best suited to carry out the invention. As it will be realized, the invention is capable of other different embodiments, and its several details are capable of modifications in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.